In the search for subterranean fluids (typically natural gas and/or petroleum), classical methods have used seismic imaging to generate an image of a subterranean region of exploration. Modern three dimensional (3D) seismic imaging has aided a great deal in developing credible models of the region of interest. Information regarding travel time of seismic signals can be used to estimate velocities of various subsurface layers, which provides information indicative of the rock type. Hydrocarbons are often trapped by geologic faults, and so the seismic images are subsequently interpreted to identify rock types which, in conjunction with geologic faults or other phenomenon, would result in an accumulation of fluids in an area in the region of study. However seismic information provides only limited information. Unless the geologic fault has a seal, fluids will not be trapped. Unfortunately, seismic survey data does not contain sufficient information to indicate the presence or absence of a seal. Other information is desirable which, if used in conjunction with seismic images, would improve the probability of finding hydrocarbons.
It is known that the field of so-called prestresses in the earth's crust is determined by two systems of external forces, namely, gravitational and tectonic forces. The joint action of these forces can result in compaction as well as in unloading of rock masses with respect to their normal gravitational or lithostatic stress condition. The existing theory describing stress fields in the earth's crust is absolutely correct for elastic and continuous media. At the same time information derived from a number of wells, drilled down to 10 km, points to the presence of fractures in sedimentary and crystalline parts of the earth's crust. Since there is little reason to expect presence of spatially localized fractures, we can affirm that any real rock unit is bounded by a closed system of fractures and represents a discrete system. Taking into account a well-known concept that any basin represents a tectonically young and consequently active structure, these are all reasons to consider any unit of the sedimentary cover as a part of discrete dynamic system. To date, no one has thought to use information pertaining to stress in the earth's crust as part of a method for determining the presence of fluid in a subterranean formation. My invention uses seismic imaging information in conjunction with formation stress information to improve the reliability of seismic exploration for hydrocarbons.
The publication of J. D. Byerlee ("Friction of Rocks", Pure Applied Geophysics, 116 (1978), pp. 615-626) is useful for estimating parameters of stress condition of fractured and discrete rock units. He has found that the maximum differential stress (the difference of main stresses) in the upper part of the earth's crust is limited by the shear strength of fractured rocks. Laboratory investigations show that the sliding friction on the surface of fractured rocks exists until the external load reaches the critical value of brittle failure. At the same time the strength of such a discrete rock unit is determined by cohesion of fractures on its surface and does not practically depend on the elastic moduli of the rock, temperature, strain value, and the type of sliding surface. It is determined that the cohesive force .tau. is a linear function of normal load .sigma..sub.n (Byerlee's law): EQU .tau.=0.85.sigma..sub.n 3&lt;.sigma..sub.n &lt;200M.PI.a (1)
or in the terms of main stresses, the limit value of horizontal stresses down to the depths of 5 km is estimated to be EQU .sigma..sub.1 .apprxeq.5.sigma..sub.3 .sigma..sub.3 &lt;110M.PI.a(2)
In other words the main component of stress condition variation in discrete media is associated with the change of vertical load. Insignificant movement in the base of a sedimentary basin (for example, in the crystalline basement) results in considerable change of the horizontal component of stress. The strength limit of a rock unit is rapidly reached, and all the sedimentary cover is set in motion continuously approaching the isostatic equilibrium. Thus, it is reasonable to suggest that real sedimentary formations have a high degree of mobility in vertical direction. As time of stress relaxation characteristic for discrete media is significant, the stress distribution in the basin has distinctive contrasting character and is governed by the principles of block dynamics.
The most fundamental theoretical results in the field of elastic wave propagation in prestressed media were derived by M. A. Biot (textbook, Mechanics of Incremental Deformations, New York, 1965). Biot introduced a new type of strain, strain of solid rotation. Its introduction is related to non-hydrostatic character of prestress field in the medium (i.e. horizontal and vertical components of the stress field are not equal to each other). In this case each unit volume of the medium has been rotated through some angle with respect to its initial position in the unstressed medium. As a result additional strains appear in the wave field, and their amplitudes are directly proportional to the difference between horizontal and vertical components of the prestress field (.sigma..sub.1 .quadrature..sigma..sub.3). A number of researchers studied the equations derived by M. A. Biot and came to the conclusion that the influence of initial stresses on traveltime and, especially, amplitude parameters of seismic waves, can be very significant, and the degree of this influence is proportional to the ratio P/.mu. (where P is pressure and .mu. is the shear modulus).
A significant number of publications are related to investigation of the stress condition of the earth with the aid of seismic methods. But practically all these investigations were based on the analysis of traveltime parameters and were reduced to attempts to explain complex distribution of velocity parameters by the influence of lithostatic pressure. In this case the stress condition of the subsurface is taken into account indirectly using theoretically and experimentally derived relationships between velocity and external load. In low-pressure regions the amplitudes and frequencies of the reflected signals completely depend on the density of fractures and the load applied to them. And the earth material (its elastic moduli) significantly influences the amplitude parameters of the signals only at the loads close to the strength limit of discrete media.
What is needed then is a method for improving the ability to discover hydrocarbons in a subterranean formation and which makes use of available data, an in particular seismic data which is likely to be collected as part of a exploratory effort in any event.